Q.BOD of waste water is estimated by measuring the amount of:

BOD stands for Biochemical Oxygen Demand. It measures the amount of oxygen required by microorganisms to break down the organic matter present in a water sample. The key point is that BOD specifically targets biodegradable organic matter -- not all organic matter, because some organic compounds resist microbial breakdown. In the test, a water sample is incubated in the dark at 20°C for 5 days, and the drop in dissolved oxygen is measured. That drop tells you how much oxygen was consumed by the microbes as they decomposed the biodegradable waste.

• Option (A) is wrong because BOD does not measure total organic matter; it only measures the portion that microbes can decompose.
• Option (B) names the target of the process (biodegradable organic matter), but the test itself does not weigh or quantify that matter directly -- so it is not what BOD is estimated by measuring. …

Biochemical Oxygen Demand, or BOD, is one of the most important parameters used to assess the pollution level of water bodies. When organic waste — such as sewage, agricultural runoff, or industrial effluents — enters water, it becomes food for bacteria and other microorganisms. These microbes break down the organic matter through aerobic respiration, using dissolved oxygen from the water. The more organic waste present, the more oxygen is consumed. BOD is essentially a measure of that oxygen consumption over a fixed period, typically five days at 20°C.

The key idea is that BOD specifically targets biodegradable organic matter. Not all organic matter in water can be broken down by microbes — some is resistant or toxic. BOD focuses only on the portion that microorganisms can decompose. This is why the test involves incubating a water sample in the dark (to prevent photosynthesis from adding oxygen) and measuring the drop in dissolved oxygen levels. The greater the drop, the higher the BOD, and the more polluted the water.

Now, looking at the options given:

• Total organic matter — This is too broad. BOD does not measure non-biodegradable or inert organic compounds.
• Biodegradable organic matter — This is what BOD indirectly measures, but it does so by measuring oxygen consumption, not the matter itself. …

Separate what BOD targets from what BOD actually reports -- it targets biodegradable organic matter, but the number read off the test is the oxygen the microbes used up doing that job, not the matter itself. Keeping 'target vs. measured quantity' as two separate idea …